What about a proper image format with support for multiple resolutions? Similar to the way it's done in SVC (http://en.wikipedia.org/wiki/Scalable_Video_Coding)
But image formats are hard, since they require an eco system, investment from large cos, etc. Maybe in a few years...
> A subset video bitstream is derived by dropping packets from the larger video to reduce the bandwidth required for the subset bitstream. The subset bitstream can represent a lower spatial resolution (smaller screen), lower temporal resolution (lower frame rate), or lower quality video signal.
Inventing a new protocol if the problem can be solved by serving the proper markup, seems misguided.